Cyclic polycarbonate oligomers are very useful in the preparation of polycarbonates. For example, U.S. Pat. Nos. 3,155,683 and 3,386,954 disclose the preparation of low molecular weight cyclic aromatic carbonate polymers and their subsequent conversion to linear polycarbonates. Furthermore, U.S. Pat. No. 4,644,053 discloses cyclic oligomer compositions which are especially suitable for conversion to high molecular weight linear polycarbonates because of their very low inherent viscosity. For example, these mixtures of cyclic oligomers can be simultaneously polymerized with an initiator and molded to produce polycarbonate articles having the highly desirable physical properties possessed by polycarbonates prepared by conventional methods.
However, one drawback associated with polycarbonate preparation from cyclic oligomers is the chemical instability of these oligomers in the melt phase, i.e., in a molten state prior to conversion to higher molecular weight linear polycarbonates. The oligomers often undergo autopolymerization at elevated temperature above about 250.degree. C., even in the absence of a polymerization initiator. It is believed that impurities present in the cyclics or introduced from the environment act as substitute initiators which induce polymerization. prior to utilization of the mixture. High initial viscosities prevent the rapid flow of the cyclic oligomer mixture into mold cavities, thereby causing considerable delays in mold processing operations.
The washing of the cyclic oligomeric mixture with a dilute solution of an acid appears to eliminate some of the impurities which cause instability in the mixture. However, even after multiple washes, residual impurities remain which continue to cause slow polymerization with an accompanying increase in viscosity, especially at temperatures of about 300.degree. C.
U.S. Pat. No. 4,814,428 discloses a method of increasing the thermal stability of cyclic carbonate oligomers by treating these cyclic oligomers with an effective amount of anhydrous hydrogen halide in an effort to neutralize any impurities which may result from the reaction used in forming cyclic oligomers (i.e., usually phenoxideterminated linear carbonate oligomers). However, anhydrous hydrogen halide is extremely corrosive and thus is frequently not commercially viable.
It is also known in the art that the melt stability of cyclic polycarbonate oligomers may be improved when phenols, diaryl carbonates with electron withdrawing groups, lewis acids, or phosphates are introduced as additives. However, additives have been found to decrease the thermal and hydrolytic stability of the final polymer.
Therefore, the present invention provides a method of preparing melt stable cyclic polycarbonate oligomers which is neither corrosive nor requires any additives.